Gridded ion sources are described in an article by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745, which is incorporated herein by reference. The ion sources described therein use a direct-current discharge to generate ions. It is also possible to use a radiofrequency discharge to generate ions, as shown by U.S. Pat. No. 5,274,306--Kaufman et al.
Typical ion optics for gridded ion sources are also described in the aforesaid article by Kaufman, et al. An improved ion optics design is described in U.S. Pat. No. 4,873,467--Kaufman, et al., which as incorporated herein by reference. The problems addressed in this patent are basic to ion optics: need to maintain the apertures in different grids in alignment while the grids and supporting members can vary in temperature, reach different equilibrium temperatures, and, at least for the grids, can have significant temperature variations within a part at equilibrium conditions.
Some specific grid temperatures are given in a chapter by Kaufman in a chapter beginning on page 265 of Advances in Electronics and Electron Physics, Vol. 36 (L. Marton, ed.), Academic Press, New York, 1974. The center of the screen grid is typically at 400 to 500.degree. C. during operation, while the center of the accelerator grid is 50 to 100.degree. C. cooler. The edges of the grids operate at 100 to 300.degree. C. cooler than the centers of the grids. Starting operation from ambient temperatures thus involves large temperature differences and gradients.
The temperature differences and variations are aggravated by the poor heat transfer in a vacuum environment, i.e., the heat transfer between parts bolted or riveted together is usually close to the heat transfer that would occur due to radiation alone. For industrial applications of ion sources, it is particularly important that routine assembly not depend on careful hand-eye coordination or the use of expensive and complicated instrumentation.
While the use of a design described in the aforesaid U.S. Pat. No. 4,873,467 is a considerable improvement over prior art in regard to maintaining alignment with varying temperatures, there are still serious problems. Using supporting members of normal flatness tolerances, large clamping forces are required to assure proper contact of parts. These forces are sufficient to plastically deform grids in the contact regions upon which the alignment depends, thereby degrading the precision of that alignment.
In some cases, positive contact of the insulator with adjacent parts is lost at some point in the startup-cooldown thermal cycle, resulting in rotation of that insulator. With a sufficient number of such cycles, a portion of the insulator that is coated with sputtered material can be rotated enough to cause electrical shorting of the ion optics.